1. Field of the Invention
The present invention relates to an electric motor drive apparatus, and in particular to an electric motor drive apparatus having a function of detecting whether or not an electromagnetic contactor for connecting and cutting off power supply has been welded, using an AC/DC converter for converting input AC voltage to DC voltage.
2. Description of the Related Art
There is a known method in the field of electric motor control apparatuses for driving machine tools, industrial machines, robots, etc., in which an electromagnetic contactor is installed in the input power line, and is turned off in order to isolate the electric motor control apparatus from the power supply when an emergency stop or an alarm occurs, to thereby stop the supply of energy from the power supply and suspend the operation of the electric motor (e.g., Published Japanese Patent No. 3506590 (JP 3506590 B)).
According to this conventional method, there is a risk that when an excessive current flows through the electromagnetic contactor, the contacts are welded, and therefore the electromagnetic contactor is unable to cut off the connection between the power supply and the electric motor control apparatus. To deal with this, it is a common that when a weld in the electromagnetic contactor occurs it is checked by monitoring auxiliary contacts of the electromagnetic contactor by turning on and off the contactor in a state where the machine is suspended without consuming energy (e.g., Published Japanese Patent Application Laid-open No. 2007-181885 (JP 2007-181885 A).
FIG. 1 shows a block diagram of a conventional electric motor drive apparatus. A conventional electric motor drive apparatus 1000 includes an AC/DC converter 101 for converting three-phase (R-phase, S-phase, T-phase) alternate current (AC) voltage supplied from an AC power supply 20 via a reactor 108 into direct current (DC) voltage; a smoothing capacitor 105 for smoothing DC voltage; and a DC/AC converter 110 for converting DC voltage to AC voltage for driving an electric motor 30. Interposed between the AC power supply 20 and the AC/DC converter 101 is an electromagnetic contactor 104. The electromagnetic contactor 104 is turned off at emergency stop or at the time of alarm generation to separate the motor control apparatus 1000 from the AC power supply 20 and stop energy supply from the AC power supply 20 to the motor control apparatus 1000.
The AC/DC converter 101 includes a plurality of power devices Tr1 to Tr6 and a plurality of diodes D1 to D6 connected in inverse parallel with these devices. The multiple power devices Tr1 to Tr6 are controlled by PWM switching in accordance with switching commands from a control circuit 102. The electromagnetic contactor 104 has auxiliary contacts 201, the presence or absence of a weld in the electromagnetic contactor 104 is detected by a detecting circuit 202. The result of detection of the presence or absence of a weld by the detecting circuit 202 is transmitted to a weld detection determining circuit 203, from which the determination result on electromagnetic contactor welding, i.e., whether or not the electromagnetic contactor 104 has been welded, is output.
There is a known device which includes a booster circuit arranged between a relay and an inverter, a capacitor connected in parallel with the inverter and a means of detecting voltage across the terminals of the capacitor, and determines that a weld of the relay is occurring if the terminal voltage of the capacitor is higher than that before boosting when the output from the booster circuit has been increased to the maximum set voltage with the relay opened (e.g., Published Japanese Patent No. 4121972 (JP 4121972 B)). However, it is necessary in this method to hold the value of the DC link voltage before boosting and the trouble of boosting the output to the maximum set voltage.
As above, in the conventional technologies, the electromagnetic contactor equipped with auxiliary contacts is needed, and an additional circuit such as a detecting circuit, a weld detection determining circuit or the like needs to be provided, externally or in the electric motor drive apparatus, in order to monitor the auxiliary contacts, which leads to increased costs.